Air slides are frequently used for moving powdered materials through a duct or conduit at desired or controlled rates of flow. The duct is divided into two positions by an air-permeable membrane supporting the material and means are provided for maintaining air under pressure against the underside of the membrane. The air passing through the membrane fluidizes the material and enables it to flow by gravity if the duct is tilted at an appropriate angle.
Air is supplied to the plenum chamber at one end of the duct and the membrane is generally clamped between the two halves of the duct, each half having opposed outwardly extending flanges for such clamping purposes. U.S. Pat. No. 2,676,851 issued Apr. 27, 1954 to Sylvest is a typical example of what has just been described.
In some instances, instead of a diaphram or membrane separating the plenum chamber from the rest of the duct, a perforated deck is employed for this purpose. The pressure of the air passing through the orifice may be varied, for instance by using two plates or decks, each with orifices, the effective size of which may be varied by sliding the plates with respect to each other so that portions of the apertures are closed. An example of this construction is shown in U.S. Pat. No. 3,475,058 to Sanders, issued Oct. 28, 1969.
The Sanders patent also provides for raising and lowering the lower deck to provide lateral grooves for passage of air and in addition, there is provided a narrower central plenum chamber through which air at different pressure may pass into the material being conveyed. The articles being conveyed are referred to as "objects such as sheets of paper cardboard, or hardboard, strips of materials, or bulky articles such as cartons, or boxes, or granular materials". Most of these materials would be incapable of being fluidized, and the provision of the slots might be effective when solid objects are being moved, without likelihood of leakage into the plenum chamber. When pulverulent material, such as cement, is to be moved, the efficacy of apparatus such as shown in Sanders is seriously doubted. At any rate, it would be necessary to build up substantial pressure in the plenum chamber before the lower deck was lowered to enable air to pass through the lateral slots. It would also be necessary to close the slots before the pressure was reduced or completely shut off. Consequently, operation and control would be extremely difficult.
The present invention relates to an improved conduit through which material may be moved under the propulsion of air which passes through a membrane to fluidize the material being conveyed. The plenum chamber providing a passageway for the flow of air is divided into two sections and separate air supply means communicate with each section. By interrupting the flow of air to one of the sections, the rate of flow of material may be materially reduced so that the shut-off gate at the outlet end of the conduit may be closed with less dribble or spillage of material.
This is a major consideration because if the conveyer is feeding cement, for instance, into a batcher, it is important to cut off the feed abruptly when the desired amount of cement has been weighed. Alternatively, it may be necessary to "jog" the cut off gate for final accurate cut-off control.
The provision of a separate air chamber within the main or normal air chamber makes it possible to pre-cut off either the inner or outer channel. If the inner channel communicates with approximately one third of the air membrane area, either a one-third or two-thirds final flow rate may be selected. This provides a simple and accurate two stage or slow down in the flow rate prior to final cut-off.
Other features of this invention are the means by which the life of the cloth membrane may be extended and when cleaning or replacement of the cloth is necessary, the operation may be facilitated. Thus the entire lower assembly of which the cloth is an integral part can be removed so that repair or replacement of cloth can occur at a convenient repair location. The inlet, outlet and upper chamber remain in place.
To avoid external moisture from wetting the ends of the cloth so that the cloth acts as a wick absorbing moisture and eventually wetting the inner portions of the cloth as well, an effective rubber seal is provided between the two halves of the slide where they overlap and are clamped together. This is important too because the slides are frequently located outside and are subject to rain and wind and also from water which is used to hose off or wash down the outside of the slide.